For example, Patent Literature 1 discloses a passive entry system. The system counts an elapsed time from a transmission of a request signal from a vehicle-mounted transceiver to a reception of an answer signal by the transceiver from a mobile transceiver, and locks or unlocks vehicle doors only when a value of counted time falls within an effective value of counted time. In the passive entry of Patent Literature 1, when a repeater is interposed between the vehicle-mounted transceiver and the mobile transceiver to perform a communication, the value of counted elapsed time exceeds the effective value of counted time due to a delay caused by the repeater. For that reason, with the configuration of Patent Literature 1, a vehicle theft by a method known as relay attack, which uses the repeater, can be prevented.